Exhaust gas recirculation (EGR) is commonly used in vehicles with internal combustion engines to reduce the amount of NO.sub.x produced in the engine cylinders during combustion. Depending on engine operating conditions, a certain amount of exhaust gas is added through an EGR valve to the intake air that goes to the cylinders. The dilution of the intake air charge with exhaust gas results in a lower combustion temperature and thus production of smaller amounts of NO.sub.x. EGR is usually measured as percentage of exhaust gas in the combined air and exhaust intake mixture. The amount of EGR is determined by the degree of opening of the EGR valve and the difference in gas pressure across the valve. Usually, EGR is measured with a position sensor that measures the degree of opening of the EGR valve. This type of measurement is not reliable, however, because a) deposits can partially block the valve and b) changes in back pressure result in changes in the amount of EGR. Another method of determining EGR is based on the measurement of the flow of the exhaust gas added to the intake air. This is done by measuring with two pressure sensors the pressure drop across an orifice through which the recirculated exhaust gas is passed. This method of determining EGR also has problems because the orifice can be partially blocked by deposits.
Still another method which may be used for measuring EGR is based on the measurement of the amount of O.sub.2 in the combined (intake air and EGR) mixture. For an engine controlled at the stoichiometric air-to-fuel ratio, the percentages of O.sub.2 in the exhaust gas is essentially zero. The percentage of O.sub.2 in the (intake air+EGR) mixture depends on the amount of EGR as shown in FIG. 1 and can be used as a measure of EGR. The effect of humidity in the air is indicated by showing plots for 0% and 100% humidity at a temperature of 70.degree. F. As shown, the error due to humidity decreases as the percentage of EGR increases. Correction for the error due to humidity is accomplished by measuring the percentage of O.sub.2 at zero EGR (completely closed EGR valve).
In the last 20 years, several different types of oxygen sensors based on O.sub.2 -pumping ZrO.sub.2 cells have been developed. Such oxygen-pumping is based on the fact that if a current is passed through an oxygen ion-conducting electrolyte (e.g., zirconia), oxygen is transferred (pumped) from one side of the electrolyte to the other. Such sensors have the common characteristic that their signal output is linearly proportional to the ambient oxygen partial pressure. As discussed, e.g., in "High Temperature Oxygen Sensors Based on Electrochemical Oxygen Pumping", E. M. Logothetis and R. E. Hetrick, Fundamentals and Applications of Chemical Sensors, 1986, American Chemical Society, the sensors may be of the single or double cell type.
In single-cell sensors, the same ZrO.sub.2 cell is used for both, oxygen pumping and sensing. In double-cell sensors, different ZrO.sub.2 cells are used for oxygen pumping and sensing. U.S. Pat. No. 4,547,281 to Wang is directed to a single cell device capable of sensing the concentration of oxygen in a volume. Double cell sensors are disclosed, e.g., in U.S. Pat. Nos. 4,272,329, 4,272,330, and 4,272,331 to Hetrick and Hetrick et al; 4,498,968 to Yamada et al; 4,645,572 to Nishizawa et al; and 4,487,680 to Logothetis et al. The Hetrick, Hetrick et al and Logothetis et al patents are commonly assigned with this invention. In general, in these two cell devices, one cell is used to pump a certain (variable) amount of O.sub.2 out of a cavity formed between the cells and the second cell (the sensor cell) is used to measure the reduced partial pressure of O.sub.2 inside the cavity. As described in the patent to Logothetis et al, the structure of that device has been modified to eliminate the cavity and employs only three electrodes, instead of the common four, but operates analogously to those of the '329, '330 and '331 patents discussed above.
Embodiments of our device are similar to the two cell devices in that they comprise two ZrO.sub.2 cells, which cells may define a cavity between them or be similar to the Logothetis et al structure discussed above. Our device, however, does not use one cell for oxygen-pumping and the second for oxygen-sensing as in the two cell devices described above. Rather, our invention uses both cells as O.sub.2 -pumping cells.
In general, measuring changes in the O.sub.2 concentration in an intake air/exhaust gas mixture is not trivial because the change in O.sub.2 concentration with changes in %EGR is relatively small. Of the various types of oxygen sensors, the ones based on oxygen pumping are more appropriate because they have high sensitivity to O.sub.2, weak temperature dependence and weak or no dependence on absolute gas pressure. In "Closed Loop Control of the EGR Rate Using the Oxygen Sensor", SAE International Congress and Exposition, Feb. 29-Mar. 4, 1988, Technical Paper No. 880133, M. Hishida, N. Inoue, H. Suzuki, and S. Kumargai disclose a device comprising an oxygen pump cell and a sensor cell useful to measure %EGR. For measuring small changes in O.sub.2 at high O.sub.2 concentrations, however, an O.sub.2 sensor with higher sensitivity is desirable. The present invention describes a method for measuring EGR and an EGR sensor which overcomes this problem.